[chris (OP)]

I was asked this very nice question on the radio today, which is "why does egg white turn from translucent to white when whipped?"

Before I impart what I said, an potentially mislead people (!), I'd value the perspectives of everyone here first...

[chiralSPO]

I think it's the same reason that snow is white while ice is translucent/transparent.

Light will scatter at every interface of two environments with different refractive indices. When the egg white is undisturbed, there is only the one surface. One it begins to fill with little air bubbles, there are many surfaces, each with its own reflections, refractions and dispersions--so no images can make it through, and it just appears as diffuse white light.

(note that cream does not need to be whipped to become white--it is already an emulsion of two liquids with different refractive indices)

[Bored chemist]

Two things (at least).
Scattering of light at lots of egg/ air interfaces.
Denaturing of the protein causing it to form an emulsion rather than a clear solution

[Colin2B]

Denaturing of the protein causing it to form an emulsion rather than a clear solution

Certainly the protein chains uncurl and change the consistency of the egg white allowing it to form bubbles.
I’d agree with the light reflecting idea due to many small reflecting surfaces in the mixture.

[chris (OP)]

Thanks everyone. [Breathes a sigh of relief] My on-air speculation appears to have been accurate!